This invention is directed to the measurement of the quality of a web, such as paper or textiles, and in particular, to an optical apparatus for analysing the surface of a fibrous paper or textile web.
The quality control of paper characteristics, such as strength and printability, is of prime concern to pulp and paper manufacturers. Important parameters in determining these characteristics are the dimensions of the fibers in the paper, their elasticity and their compactness. The coarseness or roughness of a paper or textile surface may be controlled in manufacture. Many methods of measuring quality which have been devised to date, do so in a non-real time situation, in that samples of the web are taken and analysed in a laboratory situation under microscope or the like. These measurements cannot be used to continuously control a process.
One method of measuring the graininess or roughness of a surface is described in U.S. Pat. No. 4,213,708, which issued on July 22, 1980; U.S. Pat. No. 4,194,840, which issued on Mar. 25, 1980; and U.S. Pat. No. 4,019,066, which issued on Apr. 17, 1977. This method consists of projecting a light spot on the surface of the web as it travels rapidly under the sensor. The light reflected by the web is detected in a single detector, the time-modulation component of the output signal from the detector is related to the graininess or roughness of the web. This method's accuracy is related to web speed and is not sensitive to small discontinuities.
A method for detecting deviations in a repetitive mesh pattern in a fabric is described in U.s. Pat. No. 3,783,296, which issued on Jan. 1, 1974. A collimated light beam is passed through the web to produce a diffracted pattern that is compared to a mesh pattern to detect any unacceptable deviation. This method requires transmission through a web having a repetitive pattern.